A traffic light normally consists of a housing, a parabolic reflector in the housing, a light source at the focus of the reflector and a lens which overlies the light source and reflector. The purpose of the parabolic reflector is to direct light from the source as a generally horizontal beam and the purpose of the lens is to distribute the transmitted light at a required intensity over a specified pattern.
Conventional traffic light lenses are usually moulded from a polycarbonate material to provide a body portion that is a segment of sphere and a mounting or sealing rim at the periphery of the body portion. An array of lenticular elements is formed on the concave inner surface of the body portion with each lenticular element being so shaped as to direct downwardly a portion of the horizontal beam from the reflector.
The construction and physical dimensions of the light source, the reflector and the holder for the light source as well as the colour of the lenses are virtually fixed by previous design and field compatability which is determined by local government user authorities.
The manufacturer is therefore obliged to achieve the high optical performance requirements within the little flexibility that may be gained in innovative lens design or the blending of lenses within specified chromaticity limits. In essence, lens design involves and optimisation of the sometimes conflicting requirements of diffusion, distribution, intensity and colour given a fixed light source.
The lens is, in fact, a refractor and, in Australia, its optical performance is specified by Australian Standard AS2144-1978 and various local government user specifications.
The required distribution of luminous intensity for traffic lights is set forth in table 3.1 of the above Australian Standard. The critical areas of this specification are the off-axis requirements. For red and green signals, the minimum distribution intensity 15.degree. down from the beam axis is 15 candelas at 25.degree. to the left and right of the beam axis. For yellow lights, the required intensity at these points is 45 candelas.
Prior art moulded lenses have an array of lenticular elements each of which has a light receiving surface that is formed as a portion of a section of a torroid The lenticular elements are arranged in rows that extend generally horizontally accross the concave inner surface of the body portion of the lens. Although such an array is usually effective to provide the required distribution pattern, the intensity of the emerging light is reduced by the thickness of the lens body and the lenticular elements.